Ultra-low emission natural gas dehydration unit with continuously fired reboiler

ABSTRACT

A natural gas dehydration system and method includes a contactor, a flash tank, and a still interconnected by a desiccant circulation system. A continuously fired reboiler is coupled to the still and the flash tank to bum the flash gas from the flash tank and heat the desiccant.

1. RELATED APPLICATION

This application claims the benefit of U.S. Provisional Pat. App. Ser. No. 61/322,022, filed Apr. 8, 2010, and entitled “NATURAL GAS DEHYDRATION UNIT WITH CONTINUOUSLY FIRED REBOILER.”

BACKGROUND

2. The Field of the Invention

This invention relates to natural gas dehydration units and, in particular, to the emission control of Volatile Organic Compounds (VOC's) and Benzene from natural gas dehydration units in remote field locations.

3. The Background Art

Natural gas from underground resources is commonly mixed with other hydrocarbons, such as ethane, propane, butane, and pentanes; water vapor; hydrogen sulfide; carbon dioxide; helium; nitrogen; etc. The gas is often transported through a network of pipelines that can stretch thousands of miles. The gas is usually processed to separate the various hydrocarbons and fluids to produce pipeline quality dry natural gas.

The Gas Processors Association (GPA) sets forth pipeline quality specifications for gas that the water content should not exceed 7 lb/MMSCF. The natural gas from underground resources usually contains a large amount of water, and can be completely saturated with water vapor. The water can cause problems to the pipeline, such as freezing at low temperatures, and forming hydrates with carbon dioxide and hydrocarbons that can clog equipment and pipes or cause corrosion.

In the cold northern regions of the United States and Canada, remote field location dehydration units are often used to remove the water vapor from the gas between the wellhead and the raw-gas gathering system pipelines. These gathering systems comprise of thousands of linear miles of pipelines in each gathering area which direct the raw gas to the gas processing and separations facility. An initial dehydration of the raw gas must be accomplished in the cold-climate regions, especially during the winter, in order to prevent water condensation and freezing within the gathering system.

One method of removing water vapor utilizes a liquid desiccant dehydrator, such as a glycol dehydrator. Glycol, which has an affinity for water, is used to absorb the water vapor from the natural gas. The natural gas and glycol are brought together in a contactor. The desiccant or glycol bearing the water out of the contactor is referred to as rich or wet. The pure or lean glycol flows from the top of the contactor down to the bottom of the contactor, absorbing water out of the gas as the gas flow from the bottom of the contactor to the top of the contactor. The water-rich glycol at the bottom of the contactor is referred to as rich or wet glycol. The rich or wet glycol is removed from the bottom of the contactor. The gas with the water vapor removed is referred to as dry gas and exits the top of the contactor to the gathering system pipeline.

Methane and other hydrocarbon compounds, including volatile organic compounds and benzene, are typically absorbed by the glycol and are found in the rich or wet glycol. A glycol flash tank can also be used to remove significant amounts of methane and other hydrocarbon compounds from the rich glycol that has been removed from the contactor by reducing the pressure of the glycol, allowing the methane and other hydrocarbons to vaporize or flash out of the liquid phase.

The gas that flashes out of the rich glycol can be used as a fuel source at the glycol regenerator. The rich or wet glycol is fed to a still or distillation column which is the first stage of the glycol regeneration system. The glycol regeneration system consists of a still or distillation column equipped with a fuel-gas fired reboiler. The regenerator system vaporizes the water vapor and hydrocarbon compounds from the rich glycol using the boiling-point differences between the water and the glycol. Water has a boiling point of around 100° C. (212° F.), while glycol has a boiling point of around 204° C. (400° F.). One problem with prior art regenerators is that the reboiler runs sporadically (i.e., turns on and off), such that the glycol temperature can vary by about 10° C. (50° F.). When the reboiler is off, there is no fuel gas flowing to the burner of the reboiler. The flash gas that is being burned as fuel gas when the reboiler is on or firing is vented to the atmosphere or has to be routed to a vapor destruction combustor when the reboiler is off or not firing.

A problem with the flash gas from prior art glycol flash tanks is that the flash gas is saturated with water vapor. During winter operation, the water in the flash gas will condense in the fuel gas system and will freeze at low temperatures. Frozen water or ice in the fuel gas system piping blocks the flow of fuel gas to the burner of the reboiler and turns the reboiler off. If there is not enough heat to keep the glycol system warm, the entire unit will freeze up and shut down. For this reason, the flash tank in the prior art units are by-passed during the winter months and the flash gas is vented to atmosphere or routed to a vapor destruction combustor.

Dehydration systems also commonly use a jet-gas system or gas-driven pumps which requires a large mass flow of dry gas to circulate hot glycol as a heating fluid in the winter. The jet gas and power gas significantly contribute to the overall VOC and Benzene emissions of the glycol dehydration unit.

Enhancement methods to dehydration systems often involve lowering the pressure in the system to increase stripping, using a vacuum to lower the entire still pressure, using stripping gas, using a recoverable hydrocarbon solvent, or withdrawing partially condensed vapors from the bulk liquid in the reboiler. The use of stripping gas significantly contributes to the overall VOC and benzene emissions of the glycol dehydration unit.

In addition, cold climates require more thorough and expensive glycol dehydration. Furthermore, new environmental regulations require the removal of BTEX (benzene, toluene, ethylene and xylene) compounds from the still vents of natural gas dehydrators.

Improving the dehydration process is an ongoing endeavor.

SUMMARY OF THE INVENTION

It has been recognized that it would be advantageous to develop an ultra-low emission glycol dehydration unit. In addition, it has been recognized that it would be advantageous to develop a dehydration unit that continuously utilizes all of the flash gas at the burner of the reboiler; maintains glycol temperature; eliminates the jet-gas system and power-gas pump for hot glycol circulation; uses a flash gas contactor to provide usable fuel gas to the reboiler, even during the winter; and utilizes the existing glycol pump to circulate hot glycol as a heating fluid to prevent freezing of piping and equipment during the winter that can be bypassed in the summer.

An embodiment of the invention provides a natural gas dehydration system including a main desiccant-to-gas contactor, a desiccant flash tank, and flash-gas contactor, and a desiccant regeneration system interconnected by a desiccant circulation system. Dry desiccant (such as pure or lean tri-ethylene glycol or TEG) enters the main contactor along with wet gas to absorb water vapor and leave the contactor as wet desiccant (such as rich TEG). The wet desiccant enters and leaves the flash tank with flash gas separating in the flash tank. The wet desiccant enters the regeneration system with the water vapor vaporizing, and leaves as dry desiccant returning to the main contactor. A continuously fired reboiler is coupled to the regeneration system and the flash tank to burn the flash gas from the flash tank and heat the desiccant, thus regenerating the desiccant to a relatively pure state.

In accordance with a more detailed aspect of the present invention, the system may include a flash gas contactor disposed in relation to the flash tank and coupled to the dry desiccant.

An embodiment of the present invention provides a method for dehydrating natural gas, including circulating a desiccant (such as TEG) between a contactor, a flash tank and a still with a reboiler. Wet gas is introduced into the contactor with dry desiccant (such as lean TEG) absorbing water vapor from the wet gas resulting in a wet desiccant (such as rich TEG) and dry gas. Flash gas is extracted from the wet desiccant in the flash tank. The water vapor is removed from the wet desiccant in the still by heating the wet desiccant to vaporize the water vapor resulting in the dry desiccant. The dry desiccant is recirculated from the reboiler of the regeneration system to the contactor. The reboiler is continuously fired with the flash gas from the flash tank. Since the reboiler is continuously fired, sufficient stripping gas is generated from the wet glycol in the reboiler; thus, additional or supplemental stripping gas is not required to adequately regenerate the glycol.

In accordance with a more detailed aspect of the present invention, the method includes flowing dry desiccant to a flash gas contactor on the flash tank, thus absorbing water from the flash gas and rendering the flash gas as a usable fuel gas during winter operation and effectively destroying the flash gas without venting the gas to atmosphere and without requiring a waste-gas combustor for VOC and benzene destruction.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawing. Understanding that the drawing depicts only typical embodiments of the invention and is, therefore, not to be considered limiting of its scope, the invention will be described with additional specificity and detail through use of the accompanying drawing in which:

FIG. 1 is a process flow diagram of a natural gas dehydration system in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

It will be readily understood that the components of the present invention, as generally described and illustrated in the drawing herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the system and method of the present invention, as represented in the drawing, is not intended to limit the scope of the invention, but is merely representative of various embodiments of the invention. The illustrated embodiments of the invention will be best understood by reference to the drawing, wherein like parts are designated by like numerals throughout.

As illustrated in FIG. 1, a natural gas dehydration system, indicated generally at 10, in an example implementation in accordance with the invention is shown for dehydrating natural gas. Such a system can be used in the field at remote operations adjacent one or more well heads for processing natural gas prior to transporting in a pipeline. Alternatively, the system can be used with a plant and can vent hydrocarbon vapors to a relief or fuel gas system. The system can be an ultra-low emission glycol dehydration unit that can sufficiently dehydrate raw, compressed natural gas to less than 7 lbs water/MMSCF gas with total hydrocarbon (THC) emissions of less than six tons per year. In contrast, normal THC emissions are 20 to 80 tons per year. In addition, the system can provide an ultra-low emission rate and sufficient dehydration for up to 12 MMSCFD of wet gas at 300 PSIG operating pressure, or up to 40 MMSCFD of wet gas at 1000 psig operating pressure. The system can be an absorption type dehydration system using a liquid desiccant, such as glycol or tri-ethylene glycol (TEG).

Generally speaking, the system 10 can include a contactor 14, a flash tank 18, and a regeneration system 22 with a still 26, an overhead vapor condenser 30 and a reboiler 34. A desiccant or TEG circulation system 58 can interconnect the various components with pipe or tubing. The contactor 14 can be coupled to a wet gas source 42, such as a compressor discharge, and a lean tri-ethylene glycol (TEG) source, such as the still 26 or regenerator 22. In addition, the contactor 14 is coupled to a dry-gas storage, such as the pipeline 46, and a rich TEG outlet that can be coupled to the still 26 or regenerator 22. Dry or lean TEG enters the contactor 14 along with wet gas with the TEG absorbing water vapor from the wet gas. After absorbing the water vapor, the TEG becomes wet or rich TEG and accumulates at the bottom of the contactor 14 where it leaves or is withdrawn. The gas with the water vapor removed becomes dry gas and leaves or is withdrawn from the contactor 14. Thus, lean TEG enters the contactor 14, absorbs water vapor and leaves the contactor as rich TEG. Similarly, wet gas enters the contactor 14, has its water vapor absorbed by the TEG, and exits the contactor as dry gas. The wet gas may first pass through an inlet gas separator 50 coupled between the gas source 42 and the contactor 14. The dry gas leaving the contactor 14 and the lean TEG entering the contactor can pass through a gas/glycol heat exchanger 54 which superheats the dry gas and cools the lean TEG.

A pump 58 can be coupled to the TEG circulation system to pump lean TEG into the contactor 14 and rich TEG out of the contactor. The wet TEG is withdrawn from the contactor 14 and directed to the flash tank 18 where flash gas separates from the wet TEG. The flash tank 18 can be coupled to rich TEG outlet of the contactor 14, and can have a rich TEG outlet and a flash gas outlet. The rich TEG can pass through a glycol/glycol heat exchanger 62 along with lean TEG from the still 26 on the way to the contactor 14 where the rich TEG temperature is increased and the pressure decreased. For example, the rich TEG temperature can increase between about 37° C. and about 44° C. (between about 100° F. and about 110° F.), such as from between about 33° C. and about 93° C. (between about 92° F. and about 200° F.).

In addition, a flash gas contactor 66 can be coupled to the flash tank 18. The flash gas contactor 66 can be coupled to the source of lean TEG to the contactor 14 and an outlet for the flash gas. A heat-trace system with a bypass system can be coupled in-line between the lean TEG to the flash gas contactor 66. The flash gas can be coupled to a fuel gas scrubber and outlet to a fuel tank or pipeline, which in turn, can be coupled to the burner of the reboiler 34 as discussed below. The flash gas contactor 66 provides usable fuel gas to the re-boiler, even during winter operations.

The rich TEG leaving the flash tank 18 can pass through one or more filters, such as a glycol filter 70 and a glycol charcoal filter 74 to remove impurities that may clog or foul piping or equipment. In addition, the rich TEG can pass through a glycol/glycol heat exchanger 78 coupled to the lean TEG from the still 26 to the contactor 14. Again, the rich TEG temperature is increased and the pressure decreased. For example, the rich TEG temperature can increase between about 54° C. and about 60° C. (between about 130° F. and about 140° F.), such as from between about 86° C. and about 163° C. (between about 188° F. and about 325° F.). Thus, from the contactor 14 to the still 26 or regeneration system 22, the rich TEG temperature can increase between about 110° C. and about 116° C. (between about 230° F. and about 240° F.).

The rich TEG enters the still 26 and the absorbed water and hydrocarbon compounds vaporize out of the TEG. The still 26 is coupled to the rich TEG outlet of the flash tank 18. The water and hydrocarbon vapor can vent out the top of the still 26 to the overhead vapor condenser 30 that is also coupled to the dry gas leaving the contactor 14. The water vapor can be accumulated in a liquid accumulator 82 with any waste gas vented or flared, and the liquid pumped to a condensate storage tank 86.

The reboiler 34 takes TEG in the still 26, heats it, and returns it to the still 26. Heating the TEG causes the water vapor to boil off the TEG. The reboiler 34 can be coupled to the flash tank 18 and can burn the flash gas. All of the flash gas can be burned in the reboiler 34, without venting or flaring the flash gas. The reboiler 34 can be configured to preferentially consume glycol flash gas over make-up fuel gas. The reboiler 34 can be a continuously fired reboiler 34 that maintains a consistent temperature of the TEG in the reboiler 34. A control system can be coupled to the reboiler 34 to maintain a temperature of the TEG above a predetermined minimum temperature. As described above, prior art reboilers operate sporadically burning mostly dry make-up gas from the dry-gas system 46, resulting in temperature differences of up to about 10° C. (about 50° F.) in the TEG which yields un-regenerated TEG and does not consume the flash gas from the flash tank 18. Additionally, a reboiler 34 that is continuously fired does not generally require the use of supplemental stripping gas (typically dry gas injected into the reboiler) in order to sufficiently regenerate the glycol. The lean or dry TEG is withdrawn from the still 26 into a glycol surge tank, and directed back to the contactor 14 through the heat exchangers 78 and 62 and pump 58. In addition, a side-stream of lean TEG is fed from the pump 58 to the flash gas contactor 66. The pump 58 is used to circulate hot TEG as a heating fluid or heat trace to the equipment and piping that are at risk of freezing during winter operation, and can be bypassed during summer operation.

Hydrocarbon liquids are removed from the separator 50, accumulator, glycol flash tank, fuel-gas system and power-gas system.

A method for dehydrating natural gas, and for using the system described above, includes:

-   -   1) introducing wet gas with water vapor and lean tri-ethylene         glycol (TEG) into a contactor 14 and allowing the lean TEG to         absorb water vapor from the wet gas resulting in rich TEG with         absorbed water and dry gas;     -   2) extracting the dry gas and the rich TEG from the contactor         14;     -   3) introducing the rich TEG into a flash tank 18;     -   4) separating flash gas from the rich TEG in the flash tank 18;     -   5) directing the rich TEG from the flash tank 18 to a still 26         with a reboiler 34;     -   6) heating the rich TEG in the reboiler 34 to vaporize the water         and hydrocarbon compounds from the rich TEG resulting in dry or         lean TEG;     -   7) directing the dry or lean TEG from the still 26 back to the         contactor 14; and     -   8) continuously heating the TEG by continuously firing the         reboiler 34 with dry flash gas from the flash-gas contactor 66.

The temperature of the TEG in the reboiler 34 can be maintained within at least a 10° C. (50° F.) temperature range. In addition, dry TEG from the pump 58 can be circulated to a flash gas contactor 66 disposed in relation to the flash tank 18, such as during winter. The TEG can be pumped through the circulation system, and through the heat trace to the flash gas contactor 66, with a pump, and without a jet-gas system and without a dry-gas driven pump. Furthermore, the flash gas can be washed and dried, particularly in the winter, to remove moisture and heavy hydrocarbons. In addition, all of the flash gas can be burned in the reboiler 34, without venting or flaring the flash gas.

Embodiments of the present invention may be configured to sufficiently dehydrate raw, compressed natural gas to less than 7 lbs. Water/MMSCF gas with total hydrocarbon (THC) emissions of less than about 6 tons per year (normally, between about 20 and 80 tons per year) using tri-ethylene glycol as the absorbent/desiccant up to about 12 MMSCFD of wet gas at 300 psig operating pressure and up to about 40 MMSCFD of wet gas at 1000 psig operating pressure. The present invention is also applicable to remote, field-installed units with field automation. Plant installed units of the present invention may also be configured to vent hydrocarbon vapors to a relief or fuel-gas system.

One embodiment of a method for dehydrating natural gas of the present invention may include the steps of: (1) introducing wet gas 42 with water vapor and lean tri-ethylene glycol (TEG) into a contactor 14 and allowing the lean TEG to absorb water vapor from the wet gas 42 resulting in rich TEG with water vapor and dry gas 46; (2) extracting the dry gas 46 and the rich TEG from the contactor 14; (3) introducing the rich TEG into a flash tank 18; (4) separating flash gas from the rich TEG in the flash tank 18; (5) directing the rich TEG from the flash tank 18 to a still 26 with a reboiler 34; (6) heating the rich TEG in the reboiler 34 to vaporize the water in the rich TEG resulting in dry TEG; (7) directing the dry TEG from the still 26 back to the contactor 14; and (8) continuously heating the TEG by continuously firing a reboiler 34 with the flash gas from the flash tank 18. Such embodiment of a method for dehydrating natural gas of the present invention may further involve the steps of (1) circulating lean TEG from the regeneration system 22 to a flash gas contactor 66 disposed on the flash tank during winter; (2) maintaining a temperature of the TEG in the reboiler 34 within at least a 10° C. temperature range; (3) pumping the TEG with a pump 58, and without a jet gas system; (4) washing the flash gas to remove moisture and heavy hydrocarbons in the winter; and (5) burning all of the flash gas in the reboiler 34 without venting or flaring the flash gas.

Some of the benefits realized by embodiments of the present invention may include: (1) eliminates jet-gas systems for hot glycol circulation; (2) uses no-bleed power-gas level controllers; (3) a glycol flash gas contactor provides usable fuel gas to the reboiler, even during the winter; (4) reboiler fuel system designed to preferentially consume glycol flash gas over make-up fuel gas; (5) requires additional heat-exchange surface area for glycol/glycol heat exchangers for energy optimization; (6) requires additional insulation on strategic piping and equipment; (7) Glycol Still Column overhead vapors are condensed, recovered, accumulated, and pumped to storage; (8) improved hydrocarbon liquids handling system to remove liquids from separator, accumulator, glycol flash tank, fuel-gas system, and power-gas system; (9) minimizes glycol circulation to contactor; and (10) utilizes glycol pump to circulate hot glycol heat trace during winter operation and can be by-passed during summer operation.

The present invention may be embodied in other specific forms without departing from its fundamental functions or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. All changes which come within the meaning and range of equivalency of the illustrative embodiments are to be embraced within their scope. 

1. A natural gas dehydration system, comprising: a contactor, a flash tank, and a regeneration system interconnected by a desiccant circulation system with dry desiccant entering the contactor along with wet gas to absorb water vapor and leave the contactor as wet desiccant, the wet desiccant entering and leaving the flash tank with flash gas separating in the flash tank, and the wet desiccant entering the regeneration system with the water vapor vaporizing and leaving as dry desiccant returning to the contactor; and a continuously fired reboiler coupled to a still and the flash tank to burn the flash gas from the flash tank and regenerate the desiccant.
 2. A natural gas dehydration system as defined in claim 1, further comprising a flash-gas driven pump coupled to the desiccant circulation system to pump the desiccant.
 3. A natural gas dehydration system as defined in claim 1, further comprising a control system coupled to the reboiler to maintain a temperature of the desiccant above a predetermined minimum temperature.
 4. A natural gas dehydration system as defined in claim 1, further comprising a flash gas contactor disposed on the flash tank.
 5. A natural gas dehydration system, comprising: a contactor coupled to a wet gas source and a lean tri-ethylene glycol (TEG) source, and coupled to a dry gas storage and a first rich TEG outlet; a flash tank coupled to the rich TEG outlet of the contactor and coupled to a second rich TEG outlet and a flash gas outlet; a still coupled to the second rich TEG outlet; a continuously fired reboiler coupled to the still and a flash-gas contactor configured to burn the flash gas to heat the rich TEG from the second TEG outlet and vaporize the water vapor and leave lean TEG; and a TEG circulation system disposed between the contactor, the flash tank, the still and the reboiler.
 6. A natural gas dehydration system as defined in claim 5, further comprising a flash-gas driven pump coupled to the TEG circulation system to pump the desiccant, including hot desiccant for heat trace and lean desiccant to the flash-gas contactor.
 7. A natural gas dehydration system as defined in claim 5, further comprising a control system coupled to the reboiler to maintain a temperature of the desiccant above a predetermined minimum temperature.
 8. A natural gas dehydration system as defined in claim 5, further comprising a flash gas contactor disposed on the flash tank.
 9. A method for dehydrating natural gas, comprising the steps of: circulating a desiccant between a contactor, a flash tank, and a still with a reboiler; introducing wet gas into the contactor with dry or lean desiccant, the dry or lean desiccant absorbing water vapor from the wet gas resulting in a rich or wet desiccant and dry gas; extracting flash gas from rich or wet desiccant in the flash tank; removing the water vapor from the rich or wet desiccant in the still by heating the rich or wet desiccant to vaporize the water vapor resulting in the dry or lean desiccant; recirculating the dry or lean desiccant from the still to the contactor; and continuously firing the reboiler with the flash gas from the flash tank.
 10. A method as defined in claim 9, further comprising the step of circulating lean desiccant from the still to a flash gas contactor disposed on the flash tank during winter.
 11. A method as defined in claim 9, further comprising the step of maintaining a temperature of the desiccant in the reboiler within at least a 10° C. temperature range.
 12. A method as defined in claim 9, further comprising the step of pumping the desiccant as a heating fluid with a flash-gas driven pump, and without a jet-gas circulation system and without a dry-gas driven circulation pump.
 13. A method as defined in claim 9, further comprising the step of washing and drying the flash gas to remove moisture and heavy hydrocarbons in the winter.
 14. A method as defined in claim 9, further comprising the step of burning all of the flash gas in the reboiler without venting or flaring the flash gas. 